1. Field of the Invention
The present invention relates to an imaging apparatus using an image sensor such as a charge coupled device (CCD) sensor or a complementary metal oxide semiconductor (CMOS) sensor, and more specifically relates to a technique capable of suppressing image quality from being deteriorated by a foreign substance adhered to a surface of an optical low-pass filter disposed on the front side of the image sensor. In particular, the present invention relates to a technique capable of suppressing the image quality from being deteriorated by a foreign substance in a moving image capturing operation.
Further, the present invention relates to an imaging apparatus including an electronic image stabilization system that performs shake correction by electrically clipping a part of a captured image based on camera shake information.
2. Description of the Related Art
In a state where an interchangeable lens is removed from a camera body of a digital camera, a floating dust may enter an inside space of the camera body. Further, a shutter mechanism or other mechanically operable components are disposed in the camera body. Therefore, a slug may be generated in the camera body when these mechanism components operate.
If such a foreign substance (i.e., dust or slug) adheres to a surface of the optical low-pass filter disposed on the front side of the image sensor, which is an optical element configuring an imaging unit of the digital camera, a captured image may include a shade caused by the foreign substance. Namely, the quality of the captured image may be deteriorated.
When a camera is a type using a silver-halide film, the camera successively feeds the film by a predetermined amount every time when an image is captured. Therefore, it is quite rare that continuously captured images include the shade of the same foreign substance at the same position. On the other hand, the digital cameras are not configured to feed a film every time when an image is captured. Therefore, continuously captured images may include the shade of the same foreign substance at the same position.
To solve the above-described problem, there may be a method for correcting a defective pixel corresponding to a position of a foreign substance based on signals of surrounding pixels. For example, as a technique for correcting such a defective pixel, an image defect correction method discussed in Japanese Patent Application Laid-Open No. 6-105241 proposes a method to correct pixel defects of an image sensor.
Furthermore, as discussed in Japanese Patent Application Laid-Open No. 2004-242158, to simplify setting of pixel defect position information, an extension of an image file captured in a dust acquisition mode can be differentiated from that of a normally captured image. In this case, a personal computer (i.e., PC) automatically identifies a dust information image based on the extension and corrects an image to be corrected according to the obtained information.
Various techniques are conventionally available to correct a camera shake of a digital video camera. In general, the camera shake correction includes detecting camera shake information (e.g., shake amount or shake direction) of an imaging device (e.g., digital video camera) with an external sensor or through image processing, and moving a part of an optical system to cancel the camera shake, or partly clipping the image, based on the detection result.
As discussed in Japanese Patent Application Laid-Open No. 06-98246, as an external sensor, an angular speed sensor represented by a vibration gyroscope can be used to directly measure a camera shake amount of the imaging device. Further, as discussed in Japanese Patent Application Laid-Open No. 05-7327, image processing can be used to detect a motion vector of a screen based on a plurality of captured images and detect a camera-shake amount.
As a method for correcting a camera shake, a part of an imaging lens system is moved in a direction perpendicular to an optical axis to shift an image formed on an image sensor. As discussed in Japanese Patent Application Laid-Open No. 6-105241, a variable angle prism is disposed on the front side of the imaging lens system and an apical angle of the variable angle prism is moved to shift the image formed on the image sensor.
The above-described optical camera shake correction techniques are disadvantageous in cost because they require mechanical members, such as actuators and optical elements, although a dynamic range for the correction can be enlarged.
On the other hand, as discussed in Japanese Patent Application Laid-Open No. 05-7327, an image sensor having a larger size compared to an actually required image size is prepared for a moving image capturing operation and the camera shake can be corrected by clipping a part of an image obtained by the image sensor according to a detected camera shake amount (hereinafter, referred to as “electronic image stabilization”). The above-described camera shake correction based on image clipping does not require any mechanical members, and brings an effect of reducing the cost and is therefore widely used.
In the above-described situation, not only compact digital cameras but also lens-interchangeable digital cameras are recently required to record highly accurate moving images having higher resolutions.
However, as described above, the lens-interchangeable digital camera is subjected to an influence of dusts or slugs adhered to the surface of its image sensor due to various factors. Therefore, if a moving image recording is performed with the lens-interchangeable digital camera, a foreign substance may be constantly displayed during a playback (i.e., reproduction) operation of the captured moving image.
According to a conventional dust removal method for the lens-interchangeable digital camera, dust removal relevant information (e.g., dust position and size information) and image data are recorded beforehand. Subsequently, a personal computer performs image processing on captured images to remove the influence of the dust. In this case, the recorded image data contain dust components.
An electronic image stabilization system can be used to correct an image shake generated by the camera shake when a moving image capturing operation is performed. However, when the electronic image stabilization system performs correction, a relative relationship between the dust removal relevant information and an actual position of a dust on a captured image may change because an image clipping position is variable depending on each frame. Therefore, the electronic image stabilization system cannot accurately perform dust correction processing.